The research proposed here is directed toward an understanding of the molecular mechanism of general genetic recombination. This goal is approached through the study of special sites in DNA promoting a high rate of recombination in their vicinity and through the identification and study of the activity of proteins interacting with these sites. The Chi recombinational hotspots of bacteriophage lambda and its host Escherichia coli, as well as the origin of DNA transfer (oriT) of the E. coli F-factor, will be studied in the proposed research. In addition, proteins such as RecA and RecBC known to act in the pathway stimulated by these sites will be studied. Specific aims are to: 1. Eludicate the biochemical events occurring at Chi sites, 2. Locate the point of Chi-stimulated genetic exchange, 3. Determine whether Chi acts at an early or late step of recombination, 4. Survey additional Chi-related sequences for hotspot activity, 5. Determine the activity of Chi in E. coli recombination in the absence of Chi, 6. Determine the basis of the F oriT recombinational hotspot, 7. Identify a recombinational hotspot activated by the Salmonella typhimurium RecBC enzyme, and 8. Search for recombinational hotspots in other bacteria. These aims will be approached through a combination of genetic and biochemical methods. Recombination plays an important role in generating diversity among individuals in a population and, in some cases, among cells within an organism. Understanding the mechanism of recombination thus lends insight into evolution and development. Aberrations of recombination may be responsible for certain chromosomal rearrangements associated with birth defects and cancer.